It is a scene seared forever into the memory of an eyewitness :

Masked men drag the bloodied body of a man across a public square , and tie it to a make-shift cross on a metal pole .

Green string holds the body 's arms outstretched across a wood plank as blood oozes from the gunshot wound to his head .

Militiamen wrap the body 's black `` WhatsApp '' shirt with a sign in red letters that reads in Arabic : `` This man fought Muslims and detonated an IED here . ''

The eyewitness -- a man we will call Abu Ibrahim -- does more than watch . He steps closer and snaps a picture with his cell phone ; the children around him gawk at the horrific spectacle with quiet curiosity .

Abu Ibrahim asked that his identity be kept secret for fear of reprisals . His photographs document the story of a body staged to look like a crucifixion -- and to send a message -- in the northern Syrian city of Raqqa . An al Qaeda splinter group , Islamic State of Iraq and Syria -LRB- ISIS -RRB- , says the brutal display serves as a lesson to anyone who dares challenge its rule .

Three days on , the `` crucified '' body of the man and another victim were reportedly still hanging in Raqqa .

`` What they are conveying is those who oppose ISIS rule oppose God 's rule , and those who are enemies of ISIS are enemies of God and deserve the highest form of punishment possible , '' says Abbas Barzegar , assistant professor of Islamic studies at Georgia State University .

The jihadist group carried out a total of seven public executions in Raqqa on Tuesday , but only two bodies were displayed afterward , according to the Syrian Observatory for Human Rights , a London-based monitoring group .

Abu Ibrahim , a member of a recently formed anti-ISIS activist group in Raqqa , says the remaining five victims were children under the age of 18 , one of them a seventh-grade student .

The crucifixion displays began in March , when ISIS accused a shepherd of murder and theft , then shot him in the head and tied his lifeless body to a wooden cross . Video on social media showed the body leaning up against a small building painted to bear the group 's flag and name .

`` These violent acts are part of a fundamentalist revival campaign , but these forms of ancient punishment were rarely if ever seen in the Muslim world in recent centuries , '' Barzegar says . `` It has become a standard feature of fringe Islamist groups to revive these outdated practices in an effort to bring back what they believe is authentic . ''

There 's been no evidence of actual crucifixion , a painful form of execution in which victims were bound or nailed through the hands and feet to a heavy wooden cross and left to suffer until death .

All three men in Raqqa were shot in the head prior to being affixed to crosses . The displays of their bodies appear to be largely symbolic acts by ISIS followers against members of their own Sunni Muslim sect for perceived acts of treason .

`` ISIS needs to attach meaning to their killing . Simply murdering in a state of constant warfare is void of value , so they must attach a message or propaganda to what they are doing , '' Barzegar says .

As Syria 's civil war creates a power vacuum , groups like ISIS have stepped in with their own form of radical Sharia law to rule over an exhausted and terrorized civilian population . Edicts often appear overnight on inconspicuous flyers , with dire warnings :

`` All shop owners must close their stores immediately upon the announcement of prayer and go to the mosque , '' a decree posted this week reportedly reads . `` Any violators after the issuance of this announcement will face consequences . ''

According to a set of rules issued to Raqqa 's Christian minority , members of the faith must pay a special tax to the militants and may not expose crosses , repair churches , or recite prayers in the presence of Muslims , the Syrian Observatory for Human Rights reported in February .

But while crucifixion holds specific biblical resonance , the bold and brutal displays in Raqqa hold no direct correlation to Christian symbolism , Barzegar says . The ISIS victims whose bodies were strung up on crosses were all Muslim .

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After nearly a year under the repression of ISIS , Abu Ibrahim and about 20 other activists formed a campaign in April that they called `` Raqqa is Being Slaughtered Silently '' to push the vigilante group out of their hometown .

`` After we reached the solid conviction , without the shadow of a doubt , that -LRB- Raqqa -RRB- served as the stage of a horrific spectacle that deformed the real core of the Syrian revolution , '' the campaigns founding document reads , `` we decided it was about time we stood against those forces of evil . ''

ISIS reacted almost immediately to the campaign , sentencing the activists to death for `` non-belief in Islam and their advocacy of secularism , '' and offering a large cash prize for any information on their whereabouts , according to the Syrian Observatory for Human Rights .

`` It is our obligation to confront them -LRB- ISIS -RRB- and if we remain scared of them then they will rule us forever . It is true this is dangerous and we have received more than one death threat , but we are relying on the popularity of our Facebook page as protection , '' Abu Ibrahim told CNN via a choppy Skype connection .

On their Facebook page with nearly 12,000 followers , activists post updates on alleged crimes committed against the people of Raqqa and issue calls to action such as proposing a strike by store owners on Saturday to protest an ISIS tax hike .

`` Life here is very hard . People are tired and they hate everything . If you do n't close your shop during prayer time you get lashes , if you smoke you get lashed , if you say one wrong thing you can be executed . Just like that . It 's that easy for ISIS , '' Abu Ibrahim says .

The United Nations , the Syrian opposition and human rights groups have corroborated the scenes of horror in Raqqa . Earlier this year , U.N. High Commissioner for Human Rights Navi Pillay said reported mass executions in Raqqa may amount to war crimes , and in a separate report issued last month , her office documented torture and mistreatment , including repeated beatings , of prisoners at schools and hospitals controlled by ISIS .

`` So many families have had people disappear and they have no idea where they are or what happened to them . The worst part is people are too afraid to ask about their husbands or sons , '' Abu Ibrahim says .

After nearly every Friday prayer , a few of these prisoners appear in a public roundabout where dozens of onlookers stand by as charges are hurriedly read and the sentences against the accused carried out , ranging from lashes to executions . Images of the harrowing scenes often circulate on social-media sites , sometimes posted by accounts claiming to be linked to the extremist group .

`` It is like a waterfall of blood . There are more and more executions and now the children watch like they are used to it . It is a strange and exciting scene and they are not afraid to look , '' Abu Ibrahim says .

ISIS 's military offensive against even its former allies and its savage form of justice led the central al Qaeda command to disown its affiliate earlier this year , but the group 's leader , Abu Bakr al Baghdadi , vowed to remain in Syria and fight all who oppose him , even fellow jihadists .

The founders of the peaceful campaign `` Raqqa is Being Slaughtered Silently '' say they will succeed where others have failed .

`` The word is often more powerful than the bullet , and the will of the people is the most powerful of all , '' Abu Ibrahim says .

25 children killed in elementary school bombing , Syrian activists say

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An eyewitness and his pictures document a Syrian scene meant to send a message

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Bodies are strung up on crosses in the city of Raqqa

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An al Qaeda splinter group -- ISIS -- says it 's a lesson for any who dare challenge it

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An effort is underway by local activists to try to push ISIS out of Raqqa